Patterns of bird abundance along the agricultural gradient of the Pampean region

Bird and land-use surveys were conducted along the agricultural gradient covering from pure croplands through mixed-farming to pure pastoral-farming scenario in the Pampean region of Argentina. Simple regression analyses were performed to analyse the responses of bird abundance to the percentage of land devoted to crop-production. The specific variables associated to bird abundance within both the crop and pastoral-farming landscapes were identified by running redundancy analyses (RDA). The abundance of 20 out of 43 species (47%) was significantly related to the percentage of cropland, finding negative (65%), positive (25%) unimodal (5%) and bimodal (5%) responses of birds to increasing intensity of crop production. The results showed three types of negative and two types of positive responses of bird abundance to the agricultural gradient. RDA revealed the importance of crop-field types in the crop-production scenario and of cattle presence and grass height in the pastoral-farming scenario for the structuring of the grassland bird community. As most species were negatively affected by crop production, the current land-use homogenisation towards high-intensity croplands will most likely result in population decline of many birds in the region. (c) 2006 Elsevier B.V. All tights reserved.